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Old 03-10-2005, 01:54 PM
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Carb Size

This isn't exactly a question about a carb for my specific application.. But rather a theoretical idea that I had that maybe some others will give some insite on.

Ok so at WOT manifold vacuum should be 0 correct? But, if the carb is too small this would cause restriction from not being able to flow enough to accomidate the engine needs. So theoreticaly the manifold vacuum would be unable to drop all the way to 0 if I understand correctly. So from there you'd be able to go a step up on carb size until you reach 0 manifold vacuum at WOT. Does this seem to be a good way to check if a carb is too small for your application? I know it wouldn't work in telling if the carb is too large. But it gives a good insite on half of the spectrum. (being too small).

Thanks,
Matt

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Old 03-10-2005, 02:56 PM
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Silver...yep, thats basically it
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Old 03-10-2005, 03:25 PM
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Thanks.

Thought about it.. And for tellin if a carb is too large. You could use the same meathod but include A/F ratio into the mix. If you're pulling 0 vacuum at WOT then the carb is large enough. But if it runs rich due to larger jets in larger carbs then one would know to go ahead and take it a step down.

Maybe take this as a basic scenario:
A person puts a 650 carb on and uses say a perminantly mounted boost guage to keep vacuum under close watch at all times. Well they notice that they're still runnin like 5 psi of vacuum at the manifold at full throttle so now they know to step it up. So he goes out and gets a good deal on a 750 carb. Puts it on and now he's running 0 vacuum at WOT. Everything seems great until he changes his spark plugs a week later when he notices an indication of a rich mixture. After attempting to jet it down and tuning it still runs too rich because of the large unchangeable primary jets (depending on carb). Well now he knows he needs sumthin just in between. So he trades in the 750 for a 700 and it comes out great. 0 vacuum and a stoich a/f ratio. He's successfully found the proper carb for his application.

Does this theory hold up? Or am I over thinking things?
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Old 03-10-2005, 04:55 PM
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Your over thinking, as most of us do from time to time. You thoughts on pulling manifold vac under WOT are correct for showing a restriction, usually at the carb of course. Your thoughts on the jetting versus too large a carb are really a bit off though. Most times you may have to jet up on a carb that is way over-sized due to poor signal strength, therefore a "big" carb may be jetted too lean in that situation.

on the dyno we always monitor vac readings throughout the pull as well as brake specifics and compare the two. Vac goes up, brake specifics go up, a reasonable indication we are running out of air.
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Old 03-10-2005, 10:13 PM
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You won't have 0" vacuum for long at WOT with any carb. O" would not give the carb any signal. There would be nothing pulling the intake charge in.
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Old 03-10-2005, 10:33 PM
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The piston moving down the bore pulls the mixture in due to the negative pressure when the valve opens. You don't want manifold vacuum at wide open throttle.
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Old 03-10-2005, 10:41 PM
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There is still vacuum present at WOT. It might not be a lot but there has to be. That is how a carb works.

Negative pressure is Vacuum.
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Old 03-10-2005, 11:01 PM
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Your wrong in how your thinking about this, a vac signal in a manifold port shows a restriction at wide open throttle. You have airflow through the venturi/boosters causing a fast low pressure area but if all is designed properly no vac signal in the manifold. What do you think happens on a blow through supercharger application when you start to build boost. Boost is a positive pressure.

You have pressure differentials across the boosters and airbleeds but NOT in the manifold if things are sized correctly. Carburetor function is governed by Bernoulli's principle While you want these pressure differentials across the specific locations of the carburetor you do not want them in the intake itself.

The rule of thumb typically is that a carb will flow it's rated CFM at a vac drop of 1.5" HG. So if you have a 650 CFM carb on your motor and you see manifold vacuum of aprox. 1.5 then you can assume the motor is ingesting 650 cfm of air.

Last edited by Rick WI; 03-10-2005 at 11:48 PM.
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Old 03-11-2005, 08:59 PM
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I see what you are saying about pressure differentials in a boost situation. I still think even with infinite carb size, providing that the engine is in fact running, that there will still be some vacuum present.
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Old 03-11-2005, 09:43 PM
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OK here is an exact example backed up by datalogging on the dyno. I run my motor for break in on the dyno with a 750 HP Holley carb. Run it up and pull over 4.5" vacuum. Make 600 HP as it's dead at 5900. With EFI hooked up I run a 1500 CFM throttle body and have 0 vac and make 680HP at about 6800.

Granted, typical street cars are a compromise. Many may show a slight vac at wide open throttle, maybe 1.5" or even 2". It's not optimum though for all out performance. We see this a lot on dyno runs with out of the box carbs sized "reasonable" for the application. It's all about wanting to make it as easy as you can for the motor to get as much air into the engine as possible.
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Old 03-13-2005, 09:29 AM
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Fluid flow cannot occur without an "average" total pressure gradient. I stick that "average" in there to cover the anomalies which arise in the consideration of flow in an engine intake manifold. If you've been around a dyno when an engine with injector stacks was being run at constant speed (not an inertial dyno), you might have seen a mist of fuel collect above the stacks. This is indicative of a reverse flow which occurs during part of the cycle, indicating that...for a time...the manifold pressure was actually higher than atmospheric.

When measuring manifold pressure, one must be careful that the tap is in a location where dynamic pressure effects are minimal. A stagnation tube would be preferred, though few go to that trouble.

Carburetor size is determined with a steady air flow and at a constant pressure drop. The SAE drop for a 4bbl is less than that for a 2bbl, so a 500 cfm 2bbl will flow less than a 500 cfm 4bbl at the same pressure drop.

So, yes, the total pressure directly downstream of the carburetor (or any restriction) will always be less than that directly above the carburetor, whether the absolute pressure is greater than atmospheric or not. And, yes, it's desirable to minimize pressure drop across the carburetor and, ideally, the pressure signal at the venturi could be restored as the fluid expands into the manifold, but air is not an ideal fluid and this is never going to happen. With a real fluid, there will always be losses in total pressure at the entrance and exit to the venturi.
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